Progress to stable laser propelled sails

Stable flight of a laser sail is an essential requirement of beam-driven propulsion. Centauri Dreams talked to James Benford about work towards stable beam flight. It places considerable demand upon the shape of the sail and beam.

James Benford takes a hard look at where we are now in the matter of sail stability. He and his brother Gregory have analyzed it in laboratory work. There is a great deal we still don't know. There is the need for a dedicated test facility in which deep analysis and experimentation can proceed. Dr Benford is the Chairman of the Sail Subcommittee for Breakthrough Starshot. Breakthrough starshot has $100 million in funding.

Breakthrough Starshot is a research and engineering project by Breakthrough Initiatives to develop a proof-of-concept fleet of light sail spacecraft, named StarChip, capable of making the journey to the Alpha Centauri star system, 4.37 light-years away, at speeds between 15% and 20% of the speed of light, taking between 30 and 20 years to get there, respectively, and about 4 years to notify Earth of a successful arrival. The journey may include a flyby of Proxima Centauri b, an Earth-sized exoplanet that is in the habitable zone of its host star in the Alpha Centauri system. The conceptual principles to enable this interstellar travel project were described in "A Roadmap to Interstellar Flight", by Philip Lubin of UC Santa Barbara.

The project was announced on 12 April 2016 in an event held in New York City by physicist and venture capitalist Yuri Milner and cosmologist Stephen Hawking who is serving as board member of the initiatives. Other board members include Facebook CEO Mark Zuckerberg. The project has an initial funding of US$100 million to start research. Milner places the final mission cost at $5-10 billion, and estimates the first craft could launch around 2036.

Beam sailing

Some amount of beam jitter, oscillations in the location of the beam, is to be expected. But even if the beam is steady, a sail can wander off the beam and its shape can become deformed by the high acceleration. Little of the parameter space of possible sail shapes and materials, as well as beam shapes, has been explored to date.

Sail Geometries

Generally, sails without structural elements cannot be flown if they are convex toward the beam, as the beam pressure would act to compress and perhaps collapse them. On the other hand, beam pressure keeps concave shapes in tension, so conical concave shapes are a possible natural answer to sailship configuration questions, essentially a circular cone. These shapes resist sideways motion if the beam moves off-center, since a net sideways force restores the sail to its original position.

Simulations show that this passive stability works, but it requires suspending the payload below the sail for a more stable configuration. This configuration is a conical shape, perhaps spinning, with the payload hanging from the apex along a tether, which may be flexible.

Figure 1 Conical sail with payload of mass mp hanging beneath it, connected by a tether or pendulum bob of length L (Credit: Z. Manchester).

The beam pressure keeps concave shapes of sail under tensile stress, but the periphery of the sail tends to close, so the sail needs a ring at the largest radius to keep it unfurled. This ring is under compression, and subject to elastic instability.) Such a ring may be a logical place to put the payload, which Benford thinks should be distributed to avoid single point failures. Benford would much rather have it around the periphery and with some redundancy. One bit of interstellar dust can take out the chipsail.

Figure 3 A spherical sail will resemble this helium-filled aluminized Mylar balloon. The beam pressure can cause the beam-facing side of the sail to flatten, assuming a more hemisphere shape.

The spherical sail is an idealization akin to that of the "spherical cow" of theoretical physics infamy. In practice, material under about 10,000 g's acceleration will not remain spherical. It will be deformed under acceleration into an oblate shape, and therefore the spherical symmetry will cease. So will the stability guarantee. The beam pressure causes the beam-facing side of the sail to flatten, so it assumes a more hemisphere shape. Such deformation of the sphere's surface could cause significant torques on the sail, complicating analysis. This should be explored by modeling and numerical simulations.

Finally, a spherical sail maximizes cross-section to interstellar dust and gas. Compare to a shallow cone, rotated to go edge-on to the direction of travel. The spherical sail must undergo topology change to reveal the payload, and then reconfigure to become an antenna for return of data.

The spherical sail has been studied analytically and appears to be stable only inside a hollow beam, which substantially reduces the efficiency of propulsion.

Modeling Sail Flight

Models of sail stability have thus far assumed a perfectly rigid sail. To deal with the many factors involved in sail stability, simulations must relax the rigid body constraint of work up to now.

To address the beam-riding challenge, future simulation codes should include

a large variety of sail shapes and beam profiles,
multiple internal reflections,
sail vibration modes,
sail spin,
simulations must include sail material characteristics including reflection/absorption as a function of angle of incidence.
Codes must also model noise in the system, arising from factors such as perturbations due to atmospheric turbulence, laser pattern variations as well as jitter in the beam and vibrational modes of the sail. While perturbations of the sail will surely excite oscillatory motion, the sail must be in a sufficiently deep 'potential well' to remain on the beam.

Conclusion

There is much theoretical and experimental work to be done on beam-riding, an essential requirement of beam-driven propulsion. Beam-riding and sail stability analytical models have reached their limits. Computational models must be substantially improved. Such models can be validated by laboratory experiments. For such a complex problem, only experiments will be decisive.

Looking beyond the distant goal of Star Shot, beam powered sails can play a role in the coming era of interplanetary exploration and eventual commerce. The first useful beam-riding sails will come from such stability studies, which should be undertaken now. In this lower velocity and power regime the dynamics will play out. Such lessons can have great use over decades to come.